(1) Field of the Invention
The present invention relates to a method for the preservation of a biological tissue by freeze-drying and then vacuum resin impregnation using a catalyzed silicone prepolymer which is then cured to form a transparent silicone polymer, preferably a flexible polymer, in the tissue. In particular, the present invention relates to a method which uses 1,1,1-trichloroethane as the solvent for the removal of lipids or greases from the tissue and for the impregnation of the tissue with the silicone prepolymer.
(2) Description of Related Art
The education of medical students, veterinary students and allied health personnel is significantly dependent upon the use of anatomical specimens for study. Hard tissue (skeletons) comprise only one component of the specimens available for anatomical education. These are common and environmentally safe. Skeletal material, however, constitutes only a small portion of the overall body of knowledge necessary for anatomical competency. The significant need for safe and inexpensively preserved soft tissue material remains.
Anatomical models, though readily available and safe, do not account for biological variability. This problem is faced most critically in veterinary and veterinary technology programs, biology course, public nature education and educational efforts by state and federal agencies. Traditional anatomical tissues have biohazard risks associated with the preservation and maintenance process. Potential health risks exist for students, faculty and staff with hypersensitivities and allergies to the formalin preparations used for tissue preservation. Further, the increasing number of women of reproductive years in professional programs who are exposed to these potentially hazardous materials is an additional cause for concern.
The prior art has described numerous methods for preserving biological tissue. An early method was to coat chemically preserved tissues as described in U.S. Pat. Nos. 2,606,843, 2,658,836 and 2,698,809 to Fessenden. U.S. Pat. No. 3,573,082 to Fremling and 4,784,873 to Kienecker describe various preservatives. U.S. Pat. No. 5,089,288 to Berger describes paraffin impregnation of tissue. The paraffin impregnated tissue does not provide ease of handling, since it is relatively soft and easily damaged.
U.S. Pat. Nos. 4,205,059, 4,244,992, 4,278,701 and 4,320,157 to von Hagens describe a process, referred to as "plastination" wherein a water-bearing tissue is treated with an organic solvent to remove water from the tissue. The solvent is then removed using a vacuum in the presence of a fluid precursor to a resin which replaces the solvent. Excess precursor resin is removed from the tissue and the resin is cured. The preferred resins disclosed are transparent silicone polymers. The problem is that the commercial method uses acetone as the solvent for the method which is flammable and poses a health risk. The process is slow, expensive and requires an explosion-proof laboratory. The Journal of the International Society of Plastination 1, 1 to 48 (1987) and 6 11 to 45 (1992) describe the plastination process in detail. There is a need for a method which is less hazardous.
The first three (3) of von Hagens' patents suggest direct plastination of freeze-dried tissue which was not degreased prior to the plastination step (see Column 3, lines 44 to 48). The presence of lipids or grease in the tissue is not compatible with preservation of the tissue. The cells of the embalmed tissue would collapse if the solvent was removed in the von Hagens method.
U.S. Pat. Nos. 4,567,847, 4,742,690, 4,745,771 and 4,510,169 to Linner describe a method for resin impregnation of tissue wherein cryogenic temperatures are less than -140.degree. C. and very high vacuum are used to dehydrate the tissue by freeze-drying. The equipment required for such dehydration is complicated and thus expensive; however, the problem of ice crystals fracturing the sample are said to be solved by this method. It would be an improvement if a method could be developed which avoided the temperatures and resulting equipment costs associated with this method.